The present invention relates to a decoder, and more particularly, to a decoder for detecting a synchronization pattern from a land pre-pit (LPP) read from an optical disc.
An optical disc device using an optical disc recording medium is a data recording device that records data on a recording medium. An example of such an optical disc device is a DVD-R/RW system that uses digital versatile disc-recordable (DVD-R) or digital versatile disc-rewritable (DVD-RW) discs, which enable high density recording. FIG. 1 is a schematic diagram illustrating a logic format for DVD data used in a DVD-R disc or a DVD-RW disc. In the DVD data, a SYNC pattern having a length of 32 bits and recording data having a length of 1456 bits configure the data of a single frame. The frame data of a total of 26 frames, from frame 0 to frame 25, configure the data of a single sector. The 26 frames of data are divided into even frames and odd frames to configure an array of 2 columns and 13 rows of data.
In a DVD-R/RW system that handles DVD data having such a logic format, groove tracks, which function as guide grooves, are formed in the disc to enable the system to properly recognize the tracks of the disc even when data is not recorded on the disc. Each groove track has a slightly wobbled structure, which is normally referred to as a wobble. Address pits, which are referred to as land pre-pits (LPPs), are formed adjacent to the groove tracks in land tracks. The system traces the wobble and the LPPs to generate a wobble signal and an LPP signal, which include record address information of the disc or a synchronization pattern for writing data. The wobble signal and the LPP signal are decoded to retrieve the address information and the synchronization pattern in order to accurately write data to a predetermined recording area of the disc to which data has not yet been recorded.
FIG. 2 is a waveform diagram showing an example of a wobble signal and an LPP signal. The wobble signal is a signal having a constant cycle. Eight cycles of the wobble signal is allocated for a data recording area having a length corresponding to a signal frame. In other words, the wobble of the disc is formed so that the oscillation of the wobble signal is repeated eight times when the laser beam traces a track over a length corresponding to a single frame.
The LPP signal is a whisker-like signal superposed with the wobble signal and appears in the first three cycles of the wobble signal of a frame. The LPP signal is read as an analog signal, as shown in FIG. 2, and then binarized based on whether or not the whisker-like signal exists. Subsequently, a pattern having eight bits is configured for each frame. A set of three bits at the head of the eight bits represent the information required to control writing. The LPP signal does not appear at the head of every frame. Among two successive frames, such as frames 0 and 1; 2 and 3; 4 and 5; . . . ; and 24 and 25; the LPP signal appears only in either the even frame or the odd frame. Among the 26 frames that configure a single sector, the three bit pattern configures the synchronization pattern of the two head frames, which are frame 0 and frame 1. In each pair of the other frames, the three bit pattern configures record address information of the disc.
Different synchronization patterns are allocated to frame 0 and frame 1. That is, “1, 1, 1” is allocated to frame 0 and “1, 1, 0” is allocated to frame 1. Thus, when an LPP signal matching “1, 1, 1” or “1, 1, 0” is acquired, the DVD-R system determines that that position is allocated on the disc for the head of the data recording area of a single sector.
In the record address information, the three bit LPP signal is converted to one bit data in accordance with a predetermined conversion table in the manner of, for example, “1, 0, 0”→“0” and “1, 0, 1”→“1” so that the twelve frames excluding frame 0 or frame 1 configure twelve bits of data. The twelve bit data represents track information and record address information of the disc.
In the three bit data, “1” is always recorded in the first bit. In the DVD-R system, the SYNC pattern of the data subject to recording is recorded in correspondence with the first bit.
The DVD-R/RW system reads and binarizes the LPPs recorded on a disc. The DVD-R/RW uses the binarized LPP data to generate a clock for controlling data writing and to manage the record addresses on the disc. Thus, even when data has not yet been recorded to the disc, data is recorded at the proper recording position in the proper order in compliance with the logic format. However, when reproducing the LPP data from the disc, the heat generated by a laser beam may result in a missing bit or noise may cause other bit patterns to be erroneously recognized as the synchronization pattern. In such case, the synchronization pattern or address information of the disc may not be acquired properly. This would interfere with the proper writing of data.
It is an object of the present invention to provide decoder that improves the reliability of synchronization pattern detection even when an LPP is missing or when erroneous reading occurs.